The presence of extravasated erythrocytes (EE), iron (I) and fibrin (F) within coronary atherosclerotic plaques and their relation to intraluminal coronary thrombus was determined in 2958 five-mm segments of 224 major epicardial coronary arteries in 27 control subjects. Intraplaque EE were present in 10% of the segments (controls [c] = 1%, in 35% of the arteries (c = 5%), and in 86% of the patients (c = 18%); I was present in 4% of the segments (c = less than 1%), in 14% of the arteries (c = 4%), and in 56% of the patients (c = 14%); intraplaque F was present in 2% of the segments (c = less than 1%), in 17% of the arteries (c = 3%) and in 49% of the patients (c = 11%).